A storage is computer-readable media capable of storing data in blocks. Storages face a myriad of threats to the data they store and to their smooth and continuous operation. In order to mitigate these threats, a backup of the data in a storage may be created to represent the state of the source storage at a particular point in time and to enable the restoration of the data at some future time. Such a restoration may become desirable, for example, if the storage experiences corruption of its stored data, if the storage becomes unavailable, or if a user wishes to create a second identical storage.
A storage is typically logically divided into a finite number of fixed-length blocks. A storage also typically includes a file system which tracks the locations of the blocks that are allocated to each file that is stored in the storage. The file system also tracks the blocks that are not allocated to any file. The file system generally tracks allocated and unallocated blocks using specialized data structures, referred to as file system metadata. File system metadata is also stored in designated blocks in the storage.
Various techniques exist for backing up a source storage. One common technique involves backing up individual files stored in the source storage on a per-file basis. This technique is often referred to as file backup. File backup uses the file system of the source storage as a starting point and performs a backup by writing the files to a destination storage. Using this approach, individual files are backed up if they have been modified since the previous backup. File backup may be useful for finding and restoring a few lost or corrupted files. However, file backup may also include significant overhead in the form of bandwidth and logical overhead because file backup may require the tracking and storing of information about where each file exists within the file system of the source storage and the destination storage.
Another common technique for backing up a source storage ignores the locations of individual files stored in the source storage and instead simply backs up all allocated blocks stored in the source storage. This technique is often referred to as image backup because the backup generally contains or represents an image, or copy, of the entire allocated contents of the source storage. Using this approach, individual allocated blocks are backed up if they have been modified since the previous backup. Because image backup backs up all allocated blocks of the source storage, image backup backs up both the blocks that make up the files stored in the source storage as well as the blocks that make up the file system metadata. Also, because image backup backs up all allocated blocks rather than individual files, this approach does not generally need to be aware of the file system metadata or the files stored in the source storage, beyond utilizing minimal knowledge of the file system metadata in order to only back up allocated blocks since unallocated blocks are not generally backed up.
An image backup can be relatively fast compared to file backup because reliance on the file system is minimized. An image backup can also be relatively fast compared to a file backup because seeking is reduced. In particular, during an image backup, blocks are generally read sequentially with relatively limited seeking. In contrast, during a file backup, blocks that make up the content of individual files may be scattered, resulting in relatively extensive seeking.
One common problem that is encountered during successive file backups of a source storage or successive image backups of the source storage relates to the proliferation of backups over time. For example, where a source storage is backed up every day at 2:00 am to a destination storage, after one year 365 backups will exist for the source storage on the destination storage. This proliferation of backups can increase the amount of storage space needed to store the backups on the destination storage. This problem has been mitigated to some extent by compression schemes which are employed to compress data before storing the data in a backup, thus reducing the size of each backup and saving storage space. In particular, these compression schemes typically attempt compression on all data from a source storage that is targeted for backup. This attempted compression consumes memory and processing resources. However, these compression schemes have introduced a problem of wasted resources in source storages with high-entropy data, including data from files that already have a compressed data format such as an .MP3 file format or a .ZIP file format. The problem of wasted resources results from the memory and processing resources employed during attempts at compression of high-entropy data being wasted since high-entropy data cannot be further compressed effectively.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described herein may be practiced.